Failing that, The Book of Villains & Allies - NPC's of lots of levels, fully statted, who can be adversaries or aids to any party. To my mind, mankind is one of best enemies for a party - here's a chance to showcase that.
A rogues gallery book would be wonderful.
Please not Rappan Athuk, again, there are plenty of those out there for multiple systems that would be easy to convert if someone wanted to run it using 5e.
But saying that another mega/super dungeon would be great. With a cool map accessory of jpegs or png copies to export to the virtual game table of your choice. Maybe something set in an exotic location like frozen tundra, desert, jungle, or underwater.
A book of interesting magical items, artifacts, and alchemical goods would be nice too.